| The experimental site is located at Xiayang forest farm (26°48N,117°58E), northwest Fujian Province, South-Eastern China. In April1993, eight20m×30m plots were established on hill slopes (230-278m elevation) at the harvesting site. Two tree species were then planted in the eight plots as pure forest plantations with four plots of C.lanceolata and four plots of M.laosensis seedlings. The trees were spaced at2mx2m to make up150trees per plot (2500stems ha-1). The plots were separated by more than10buffer tree rows. Based on the comparison between19-year-old M.laosensis and C.lanceolata forests in above-and below-ground biomass, carbon (C) and nitrogen (N) storages, soil physical and chemical properties at0-20cm depth, the monthly dynamics of soil respiration and nitrogen mineralization, and soil microbial biomass and community composition, the objective of this study was to assess the potential impact of a land use transition from C.lanceolata to M.laosensis plantaion forest on soil biogeochemical cycle processes. The results showed that:(1) Under M.laosensis plantation, forest floor biomass, C and N storages were1.6,1.5and1.5times of that under C.lanceolata plantation, respectively. Annual litterfall biomass under M.laosensis plantation was9.50t·hm-2, which is more than that under C.lanceolata plantation (4.31t-hm"2). M.laosensis litter has a higher decomposition rate than C.lanceolata litter, due to a higher leaf litter N concentration and a lower C/N ratio. Fine root (diameter is less than2mm) biomass (0-80cm) under M.laosensis plantation was1.3times of that under C.lanceolata plantation. The results indicate that the transition of land use from C.lanceolata to M.laosensis leads to an enhanced production and C and N storages in above ground.(2) Plantings of native broadleaved tree species in C.lanceolata sites have increased soil total C stock in the0-5and5-10cm layers, however, there was no significant difference in soil total N stock between the two species. Differences in the soil total C stock between the two spcies may be attributed to the quality and quantity of above-and below-ground litter. Under M.laosensis plantaion, the concentrations of soil organic C concentrations extracted by cold water, hot water and2M KC1solutions were45.5%-77.2%,39.6%-66.9%and28.2%-42.2%significantly higher than those under C.lanceolata plantation, respectively. The concentrations of soil organic N between M.laosensis and C.lanceolata were found to be significantly different in the0-5and5-10cm layers. We also found that the concentrations of soil microbial biomass C and light fraction organic C and N at0-5cm depth under M.laosensis plantaion were significantly higher than under C.lanceolata plantation. The results indicated that the transition of land use from C.lanceolata to M.laosensis can improve soil fertility in the plantation.(3) The soil respiration rates (Rs) has changed by a range of1.17to4.57μmol·m-2·s-1in M.laosensis and from0.84to3.66μmol·m-2·s-1in C.lanceolata. We observed that the Rs exhibited a similar seasonal dynamics in the two plantations, with higher in summer and lower in winter. Annual soil respiration rate was significantly grater (11.48tC·hm-2a-1) under M.laosensis plantation than that (9.12tC·hm-2a-1) under C.lanceolata plantation. The temperature sensitivity of Rs (Q10) was lower in M.laosensis (2.05) than that in C.lanceolata (2.12), which indicated that the soil respiration in C.lanceolata was sensitive to soil temperature change. Soil temperature and moisture could explain changes in Rs of86.3%under M.laosensis plantation and91.1%under C.lanceolata plantation. Correlation analysis showed that monthly litterfall production and fine root biomass at0-10cm soil depth were correlated with Rs in the two plantations (P<0.05).(4) It was found that ammonia nitrogen (NH4+-N) was the dominant form in the mineral N pool in both M.laosensis and C.lanceolata plantation soils and account for79.0%-98.9%of inorganic N. The NH4+-N and nitraite N (NO3--N) pools were significantly higher in C.lanceolata soil than in M.laosensis soil (P<0.05). In the laboratory experiment, we found that M.laosensis soil has a significantly lower net N nitrification (0.01μg·g-1·d-1) and mineralization rate (0.28μg·g-1·d-1) than that under C.lanceolata soil (0.30μg·g-1·d-1and0.78μg·g-1·d-1,respectively)(P<0.01). Correlation analysis showed that soil net mineralization and net nitrification rates were positively correlated with soil pH, and negtively correlated with hot-water extracted organic matter and litter C/N ratio. The results showed that interspecific differences in soil pH, labile soil organic matter and litter qualitiy and quantity appear to regulate the rate of net N mineralization in the two plantations. The reforestation with M.laosensis would cause a relatively higher capacity for N rentation compared with reforestation with C.lanceolata. (5) Both of the soil microbial biomas C and N in M.laosensis were higher than that in C.lanceolata, which mainly attributed to greater monthly litter biomass in M.laosensis plantation. The M.laosensis plantation has a similar microbial community composition compared to C.lanceolata plantation. However, tree species significantly affect the content of individual lipid biomarkers. Under M.laosensis plantation, the contents of fungi PLFAs, bacteria PLFAs and actinomycetes PLFAs were all significantly higher than those under C.lanceolata plantaion (P<0.05). We also found that the ratio of gram-positive bacteria to gram-negtive bacteria in M.laosensis soil was significantly lower than in C.lanceolata soil, however, the ratio of fungi to bacteria in M.laosensis was significantly higher than in C.lanceolata. Correlation analysis showed that soil microbial community was negtively correlated with soil pH, and positively correlated with hot-water extracted organic matter and litter biomass and C/N ratio (P<0.05). Our results indicated that tree species induced differences in soil microbial community content by variations of soil pH, labile organic matter and litter qualitiy and quantity. |
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